The diagnosis, treatment and prevention of viral infections is a primary focus of many medical researchers. Although compositions and methods of diagnosing, treating and vaccinating against a number of viral infections are known, there are still a number of viruses which are difficult to detect in man and for which no effective methods of treatment or vaccination against are known. Of these, one of the most significant, of course, is HIV.
The infectious agent responsible for acquired immunodeficiency syndrome (AIDS) and its prodromal phases, AIDS-related complex (ARC) and lymphadenopathy syndrome (LAS), is a lymphotrophic retrovirus termed LAV, HTLV-III, ARV, and recently HIV as recommended by the International Committee on Taxonomy of Viruses (Ref 299). Nomenclature herein employs these recommendations to designated viruses associated with AIDS and the strains thereof. Historic references to strains, which include LAV and ARV-2, are now named HIV1 LAI and HIV1SF2, respectively.
As the spread of HIV reaches pandemic proportions, the treatment of infected individuals and prevention of the transmission to uninfected individuals at risk of exposure is of paramount concern. A variety of therapeutic strategies have targeted different stages in the life cycle of the virus and are outlined in Mitsuya and Broder, 1987, Nature 325:773. One approach involves the use of antibodies which bind to the virus and inhibit viral replication, either by interfering with viral entry into host cells or by some other mechanism. Once the viral component(s) susceptible to antibody intervention are identified, it has been hoped that antibody reactivity sufficient to neutralize the infectivity of the virus could be generated and administered to HIV-infected patients in the form of immune globulins or purified antibodies and that this passive immunization procedure would alter or reverse progression of HIV infection. In addition, it has been hoped that the vaccination of non-infected individuals with selected epitopes modified to enhance MHC interactions would provide protection from subsequent infection following exposure to HIV.
The envelope glycoproteins of most retroviruses are thought to react with receptor molecules on the surface of susceptible cells, thereby determining the virus' infectivity for certain hosts. Antibodies that bind to these envelope glycoproteins may block the interaction of the virus with the cell receptors, neutralizing the infectivity of the virus. See generally, The Molecular Biology of Tumor Viruses, 534 (J. Tooze, ed., 1973); and RNA Tumor Viruses, 226, 236 (R. Weiss et al., eds., 1982); Gonzalez-Scarano et al., 1982, Virology 120:42 (La Crosse Virus); Matsuno and Inouye, 1983, Infect. Immun. 39:155 (Neonatal Calf Diarrhea Virus); and Mathews et al., 1982, J. Immunol., 129:2763 (Encephalomyelitis Virus). To date, therapeutic strategies directed at eliciting protective immune responses in man by vaccination with HIV proteins/peptides have failed. In addition, neither high titer neutralizing antibodies recovered from HIV-infected patients nor monoclonal antibodies produced in mice have succeeded in altering the progression of HIV infection to AIDS and death. There is a need in the art to identify alternate immunological targets on HIV which will elicit immune responses that will modify the course of HIV infection.
The general structure of HIV is that of a ribonucleo-protein core surrounded by a lipid-containing envelope which the virus acquires during the course of budding from the membrane of the infected host cell. Embedded within the envelope and projecting outward are the viral encoded glycoproteins. The envelope glycoproteins of HIV are initially synthesized in the infected cell as a precursor molecule of 150,000-160,000 Daltons (gp 160), which is then processed in the cell into an N-terminal fragment of 110,000-120,000 Daltons (gp120) to generate the external glycoprotein, and a C-terminal fragment of 41,000-46,000 Daltons (gp 41), which is the transmembrane envelope glycoprotein.
For the reasons discussed above, the gp120 glycoprotein of HIV has been the object of much investigation as a potential target for interrupting the virus' life cycle. Sera from HIV-infected individuals have been shown to neutralize HIV in vitro, and antibodies that bind to purified gp120 are present in these sera, (Robert-Guroff et al., 1985, Nature 316:72; Weiss et al., 1985, Nature 316:69; and Mathews et al., 1986, Proc. Natl. Acad. Sci. U.S.A., 83:9709). Purified and recombinant gp120 stimulated the production of neutralizing serum antibodies when used to immunize animals (Robey et al., 1986, Proc. Natl. Acad. Sci. U.S.A., 83:7023; Lasky et al., 1986, Science, 233:209) and a human (Zagury et al., 1986, Nature 326:249). Binding of the gp120 molecule to the CD4 receptor also has been shown and monoclonal antibodies which recognize certain epitopes of the CD4 receptor have been shown to block HIV binding, syncytia formation, and infectivity. McDougal et al., (1986, Science 231:382) and Putney et al. (1986, Science 234:1392) elicited neutralizing serum antibodies in animals after immunizing with a recombinant fusion protein containing the carboxyl-terminal half of the gp 120 molecule and further demonstrated that glycosylation of the envelope protein is unnecessary for a neutralizing antibody response.
Shortly after HIV infection the immune system of man responds to the virus with both antibody production and cell mediated immune responses. A review of the immune responses to retroviruses has been published (Norley, S., and Kurth R., 1994: The Retroviridae, Vol E, J. A. Levy, ed., pp. 363-464, Plenum Press). Human antibodies specific for a number of HIV proteins including gp 160, gp120, p66, p55, gp 41, p32, p24, and p17 have been reported (Carlson, 1988, J. Am. Med. Assoc. 206:674). The initial antibody response in man to HIV is directed to p17 and p24, followed by gp120/160, then by gp 41, p66/55 and finally p32 (Lange 35 et al 1986, Br. Med. J. 292:228). As HIV infection progresses into AIDS antibody levels to p17 and p24 markedly fall to undetectable limits and are replaced by p17 and p24 antigenemia. Antibody titers to p32 and p55 also decline but to a lesser degree (McDougal et al 1987 J. Clin. Invest. 80:316). However, substantial amounts of antibodies to gp 160/120 persist throughout the entire course of HIV infection. During the early phases of HIV infection an elevation in total immunoglobulins is observed and this increased quantity of antibody is specific for HIV and predominantly directed to gp120, (Amadori et al., 1988 Clin. Immunol. Immunopathol. 46:342; Amadori et al, 1989, J. Immunol. 143:2146). Possible mechanisms for this HIV specific hyper gamma globulinemia have been reviewed by Barker E. et al 1995: The Retroviridae Vol 4, J. A. Levy, ed. pp 1-96 Plenum Press. Functional properties and epitopes targeted by these antibodies produced during HIV infection have been described and include epitopes which are susceptible to antibody mediated neutralization. These primary target epitopes are primarily located on the envelope protein gp160 (gp120/gp41) and the gag protein p17; for review see Levy, 1994 Am. Soc. Micro; Nixon et al, 1992 Immunol 76:515. Neutralizing antibodies to HIV envelope protein have been identified and bind to conserved and divergent domains on gp120. These include regions localized to the CD4 binding regions (Linsley et al 1988 and Thali et al, 1992); the second and third variable loop domains (Fung et al, 1992 and Haigwood et al 1990); and carbohydrate moieties (Benjouad et al, 1992 and Feizi and Larkin, 1990). Other neutralization sites have been identified on the external portion of gp 41 and a binding site on p17 (Changh et al, 1986). Early studies suggested that the presence of neutralizing antibodies lead to a more favorable clinical outcome, (Robert-Guroff et al, 1985). However, these studies employed selected sera with high neutralizing capacity against laboratory strains of HIV and not against autologous HIV isolates (Homsy et al, 1990; Tremblay and Wainberg, 1990). Subsequent investigation demonstrated that autologous antibody had little or no neutralizing activity against autologous HIV isolates (Homsy et al, 1990). The lack of susceptibility to antibody mediated neutralization in the presence of a neutralizing antibody is thought to result from the development of escape mutants that appear after seroconversion (Arendrup et al, 1992) and throughout the infection as new antibody specificities are produced. The clinical relevance of neutralizing antibodies produced as a consequence of HIV infection is unclear. However, it is clear that in spite of a vigorous immune response to HIV in individuals infected with HIV, progress to AIDS and, ultimately, death as a consequence of immune dysfunction predominates. Accordingly, new methods of treatment are sought.